Commercial practices have resulted in the production of pollutants that can contaminate the environment. For instance, modern agricultural practices rely heavily on the use of herbicides to control weed populations. S-triazine (i.e., symetric triazine) herbicides, primarily atrazine and simazine, are widely used herbicides for selective control of broadleaf weeds and some grasses in a variety of crops. Since atrazine and other s-triazine herbicides biodegrade relatively slowly in soils, label directions for the use of atrazine restrict the types of crops that can be planted to prevent carryover problems in the next growing season. For example, alfalfa and soybeans are susceptible to atrazine concentrations in soil ranging from 0.09 mg/Kg to 0.53 mg/Kg, depending on the concentration of soil organic matter.
Numerous studies on the environmental fate of atrazine have shown that atrazine is a moderately persistant compound that is transformed to CO2 very slowly, if at all, under aerobic or anaerobic conditions. It has a water solubility of 33 mg/l at 27° C. Its half-life (i.e., time required for half of the original concentration to dissipate) can vary from about 4 weeks to about 57 weeks when present at a low concentration (i.e., less than about 2 parts per million (ppm)) in soil. High concentrations of atrazine, such as those occurring in spill sites, have been reported to dissipate even more slowly.
As a result of its widespread use, atrazine is sometimes detected in water in concentrations exceeding the maximum contaminant level (MCL) of 3 μg/l (i.e., 3 parts per billion (ppb)), a regulatory level that took effect in 1992. Point source spills of atrazine have resulted in levels as high as 25 ppb in some wells. Levels of up to 40,000 mg/l (i.e., 40,000 ppm) atrazine have been found in the soil at spill sites more than ten years after the spill incident. Point source spills and subsequent runoff can result in the presence of atrazine in surface, subsurface, and ground water.
Atrazine removal from the soil environment can occur by several different mechanisms. At typical soil pH, atrazine is only very slowly chemically hydrolyzed (half life of 200 days) to produce hydroxyatrazine. A more significant degradation mechanism for atrazine in soils is microbial metabolism. Microbial degradation of atrazine has been demonstrated to occur via dealkylation, deamination, or dechlorination reactions.
For decontamination purposes, the most efficient method of transforming a contaminant into a less-harmful end product is by biostimulation or bioaugmentation (Liu et al. (1993) TibTech., 11, 344-352). Biostimulation involves supplementing the contaminated soils to change the physical state of the contaminant, thereby converting it to a bioavailable form, or supplying a nutritional supplement or co-substrate to increase the population of indigenous bacteria capable of catabolizing the contaminant. Bioaugmentation involves inoculating soils with a non-indigenous microorganism capable of catabolizing the contaminant.
The ability of introduced live cultures of atrazine-degrading bacteria to increase biodegradation has been investigated in laboratory studies. In studies done with non-sterile soil, the success of bioaugmentation was inversely related to population levels of indigenous atrazine-degrading microorganisms (Radosevich et. al., (1996) Biodeg., 7, 137-149; Struthers et al., (1998) Appl. Environ. Microbiol., 64, 3368-3375; and Kontchou et al., (1993) Proceedings of the 9th Symposium on Pesticide Chemistry, Piacenza Italy. p. 533-536. Istituto di Chimica Agraria et Ambientale, Universita Cattolica del Sacro Cuore). In sterile soils devoid of indigenous atrazine degrading bacteria, it has been reported that atrazine concentration was reduced 70% (from 20 ppm to 6 ppm) in 30 days (Fadullon et al., (1998) J. Environ. Sci. Health, B33, 37-49), or eliminated from 15 ppm in 5 days (Wenk et al, (1998) Appl. Mibrobiol. Biotechnol., 49, 624-630).